1. Field of the Invention
The invention relates Lo seismic exploration and, more particularly, to a digitally driven test signal generator for testing seismic sensors and related electronics.
2. Description of Related Art
Digital to analog converters (DACs) are well known in the art and have a number of uses. In one such use, a DAC takes a plurality of digital values, representing consecutive samples of analog signal in digital form, and converts the sequential digital sample values into a corresponding analog signal. Typically, digital to analog converters have some form of filter which smooths out the transition between discrete sample values.
Integrators are also known in the art. Some integrators are passive, in that they are made up of only components such as resistors or capacitors. Other integrators are active, using an amplifier to transfer the signal to an integration element, usually a capacitor. For low distortion, low noise applications, can active integrator is best suited. With CMOS integrated circuits, the switched capacitor integrator, and more generally, the switched capacitor filter is a commonly used circuit. In a switched capacitor circuit, a voltage is sampled onto a capacitor in one phase, and the resulting charge is transferred in a second phase, resulting in a current flow. This switched capacitor "branch" behaves much like a resistor when viewed at a low frequency. Its advantage in CMOS integrated circuits include ease of manufacturing and ease of matching to other elements.
Systems for conducting seismic exploration are well known in the art. On land, a plurality of transducers are deployed over a region and configured to receive reflections of acoustic signals from different geophysical layers beneath the surface of the earth. Seismic sensors are connected over cables to signal conditioning, digitization and digital recording equipment. When utilizing a seismic system, a strong acoustic signal is generated by, for example, setting off an explosion or by utilizing an acoustic signal generator having a relatively high power output. Reflections of the acoustic signals from the geophysical layers are then received at the seismic sensors deployed over a given area and the signals recorded, typically, for later analysis.
One problem with seismic exploration is that it frequently occurs in remote areas. Once sensors are deployed over a large area and seismic data gathered, great expense would be incurred if data were corrupted by malfunctioning sensors or electronics and a seismic survey crew needed to return again to the site, set up equipment and re-gather the data.
Seismic exploration has exacting requirements for seismic sensors and for the electronics which processes the signals derived from seismic sensors. There is therefore a need for testing equipment to ensure that both the devices and the associated electronics are functioning properly. When driving analog devices with digitally generated waveforms, it is sometimes the case that the analog device has nonlinear characteristics which adversely affect the analog device's ability to track the signal being applied to it. In addition, there are generally problems with linearity and noise when dealing with digital to analog converters.